DOCSLIB.ORG

Project 2: User Level Thread Library 15-410 Operating Systems September 19, 2007

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
Project 2: User Level Thread Library 15-410 Operating Systems September 19, 2007 Project 2: User Level Thread Library 15-410 Operating Systems September 19, 2007 1 Overview An important aspect of operating system design is organizing computations that run concurrently and share memory. Concurrency concerns are paramount when designing multi-threaded programs that share some critical resource, be it some device or piece of memory. In this project you will write a thread library and concurrency primitives. This document provides the background information and specification for writing the thread library and concurrency primitives. We will provide you with a miniature operating system kernel (called “Pebbles”) which implements a minimal set of system calls, and some multi-threaded programs. These programs will be linked against your thread library, stored on a “RAM disk,” and then run under the supervision of the Pebbles kernel. Pebbles is documented by the companion document, “Pebbles Kernel Specification,” which should probably be read before this one. The thread library will be based on the thread fork system call provided by Pebbles, which provides a “raw” (unprocessed) interface to kernel-scheduled threads. Your library will provide a basic but usable interface on top of this elemental thread building block, including the ability to join threads. You will implement mutexes and condition variables based on your consideration of the options provided by the x86 instruction set—including, but not limited to, the XCHG instruction for atomically exchanging registers and memory or registers and registers. 2 Goals • Becoming familiar with the ways in which operating systems support user libraries by providing system calls to create processes, affect scheduling, etc. • Becoming familiar with programs that involve a high level of concurrency and the sharing of critical resources, including the tools that are used to deal with these issues. • Developing skills necessary to produce a substantial amount of code, such as organization and project planning. • Working with a partner is also an important aspect of this project. You will be working with a partner on subsequent projects, so it is important to be familiar with scheduling time to work, a preferred working environment, and developing a good group dynamic before beginning larger projects. • Coming to understand the dynamics of source control in a group context, e.g., when to branch and merge. The partner goal is an important one–this project gives you an opportunity to debug not only your code deliverables but also your relationship with your partner. You may find that some of the same techniques apply. 1 3 Important Dates Wednesday, September 19th Project 2 begins Monday, September 24th You should be able to draw a very detailed picture of the parent and child stacks during thr create() at the point when the child does its first PUSHL instruction. In your design multiple pictures may be equally plausible, but it is important that you be able to draw at least one case in detail. Wednesday, September 26th You should have thread creation working well enough to pass the STARTLE test we provide. Friday, September 28th You should have thread creation, mutexes, and condition variables working well. Tuesday, October 2nd If you haven’t least begun debugging CYCLONE and AGILITY DRIL by this point, you run the risk of turning in a thread library with serious structural flaws and lacking concurrency debugging experience useful for the kernel project. Friday, October 5th Project 2 due at 23:59:59 4 Thread Library API The library you will write will contain: • Thread management calls • Mutexes and condition variables • Semaphores • Readers/writers locks Please note that all lock-like objects are defined to be “unlocked” when created. Unlike system call stubs (see “Pebbles Kernel Specification”), thread library routines need not be one- per-source-file, but we expect you to use good judgement when partitioning them (and this may influence your grade to some extent). You should arrange that the Makefile infrastructure you are given will build your library into libthread.a (see the README file in the tarball). You may assume that programs which use condition variables will include cond.h, programs which use semaphores will include sem.h, etc. 4.1 Thread Management API • int thr init( unsigned int size ) - This function is responsible for initializing the thread library. The argument size specifies the amount of stack space which will be available for use by threads created with thr create(). This function returns zero on success, and a negative number on error. The thread library can assume that programs using it are well-behaved in the sense that they will call thr init(), exactly once, before calling any other thread library function (including memory 2 allocation functions in the malloc() family, described below) or invoking the thread fork system call. Also, you may assume that all threads of a task using your thread library will call thr exit() instead of directly invoking the vanish() system call (and that the root thread will call thr exit() instead of return()’ing from main()). • int thr create( void *(*func)(void *), void *arg ) - This function creates a new thread to run func(arg). This function should allocate a stack for the new thread and then invoke the thread fork system call in an appropriate way. A stack frame should be created for the child, and the child should be provided with some way of accessing its thread identifier (tid). On success the thread ID of the new thread is returned, on error a negative number is returned. You should pay attention to (at least) two stack-related issues. First, the stack pointer should essentially always be aligned on a 32-bit boundary (i.e., %esp mod 4 == 0). Second, you need to think very carefully about the relationship of a new thread to the stack of the parent thread, especially right after the thread fork system call has completed. • int thr join( int tid, void **statusp ) - This function “cleans up” after a thread, optionally returning the status information provided by the thread at the time of exit. The target thread tid may or may not have exited before thr join() is called; if it has not, the calling thread will be suspended until the target thread does exit. If statusp is not NULL, the value passed to thr exit() by the joined thread will be placed in the location referenced by statusp. Only one thread may join on any given target thread. Other attempts to join on the same thread should return an error promptly. If thread tid was not created before thr join(tid) was called, an error will be returned. This function returns zero on success, and a negative number on error. • void thr exit( void *status ) - This function exits the thread with exit status status. If a thread other than the root thread returns from its body function instead of calling thr exit(), the behavior should be the same as if the function had called thr exit() specifying the return value from the thread’s body function. Note that status is not a “pointer to a void.” It is frequently not a pointer to anything of any kind. Instead, status is a pointer-sized opaque data type which the thread library transports uninterpreted from the caller of thr exit() to the caller of thr join(). • int thr getid( void ) - Returns the thread ID of the currently running thread. • int thr yield( int tid ) - Defers execution of the invoking thread to a later time in favor of the thread with ID tid. If tid is -1, yield to some unspecified thread. If the thread with ID tid is not runnable, or doesn’t exist, then an integer error code less than zero is returned. Zero is returned on success. Note that the “thread IDs” generated and accepted by your thread library routines (e.g., thr getid(), thr join()) are not required to be the same “thread IDs” which are generated and accepted by the thread- related system calls (e.g., thread fork, gettid(), cas2i runflag()). If you think about how you would implement an “M:N” thread library, or a user-space thread library, you will see why these two 3 name spaces cannot always be the same. Whether or not you use kernel-issued thread ID’s as your thread library’s thread ID’s is a design decision you will need to consider. However, you must not aggressively recycle thread ID’s, as this significantly reduces the utility of, e.g., thr yield(). 4.2 Mutexes Mutual exclusion locks prevent multiple threads from simultaneously executing critical sections of code. To implement mutexes you may use the XCHG instruction documented on page 3-714 of the Intel Instruction Set Reference. For more information on the behavior of mutexes, feel free to refer to the text, or to the Solaris or Linux pthread mutex init() manual page. • int mutex init( mutex t *mp ) - This function should initialize the mutex pointed to by mp. The effects of using a mutex before it has been initialized, or of initializing it when it is already initialized and in use, are undefined (and may be startling). This function returns zero on success, and a negative number on error. • int mutex destroy( mutex t *mp ) - This function should “deactivate” the mutex pointed to by mp. The effects of using a mutex from the time of its destruction until the time of a possible later re-initialization are undefined. If this function is called while the mutex is locked, it should immediately return an error. This function returns zero on success, and a negative number on error. • int mutex lock( mutex t *mp ) - A call to this function ensures mutual exclusion in the region between itself and a call to mutex unlock().
Load more

Recommended publications
  • An Introduction to Linux IPC
    An introduction to Linux IPC Michael Kerrisk © 2013 linux.conf.au 2013 http://man7.org/ Canberra, Australia [email protected] 2013-01-30 http://lwn.net/ [email protected] man7 .org 1 Goal ● Limited time! ● Get a flavor of main IPC methods man7 .org 2 Me ● Programming on UNIX & Linux since 1987 ● Linux man-pages maintainer ● http://www.kernel.org/doc/man-pages/ ● Kernel + glibc API ● Author of: Further info: http://man7.org/tlpi/ man7 .org 3 You ● Can read a bit of C ● Have a passing familiarity with common syscalls ● fork(), open(), read(), write() man7 .org 4 There’s a lot of IPC ● Pipes ● Shared memory mappings ● FIFOs ● File vs Anonymous ● Cross-memory attach ● Pseudoterminals ● proc_vm_readv() / proc_vm_writev() ● Sockets ● Signals ● Stream vs Datagram (vs Seq. packet) ● Standard, Realtime ● UNIX vs Internet domain ● Eventfd ● POSIX message queues ● Futexes ● POSIX shared memory ● Record locks ● ● POSIX semaphores File locks ● ● Named, Unnamed Mutexes ● System V message queues ● Condition variables ● System V shared memory ● Barriers ● ● System V semaphores Read-write locks man7 .org 5 It helps to classify ● Pipes ● Shared memory mappings ● FIFOs ● File vs Anonymous ● Cross-memory attach ● Pseudoterminals ● proc_vm_readv() / proc_vm_writev() ● Sockets ● Signals ● Stream vs Datagram (vs Seq. packet) ● Standard, Realtime ● UNIX vs Internet domain ● Eventfd ● POSIX message queues ● Futexes ● POSIX shared memory ● Record locks ● ● POSIX semaphores File locks ● ● Named, Unnamed Mutexes ● System V message queues ● Condition variables ● System V shared memory ● Barriers ● ● System V semaphores Read-write locks man7 .org 6 It helps to classify ● Pipes ● Shared memory mappings ● FIFOs ● File vs Anonymous ● Cross-memoryn attach ● Pseudoterminals tio a ● proc_vm_readv() / proc_vm_writev() ● Sockets ic n ● Signals ● Stream vs Datagram (vs uSeq.
    [Show full text]
  • Multithreading Design Patterns and Thread-Safe Data Structures
    Lecture 12: Multithreading Design Patterns and Thread-Safe Data Structures Principles of Computer Systems Autumn 2019 Stanford University Computer Science Department Lecturer: Chris Gregg Philip Levis PDF of this presentation 1 Review from Last Week We now have three distinct ways to coordinate between threads: mutex: mutual exclusion (lock), used to enforce critical sections and atomicity condition_variable: way for threads to coordinate and signal when a variable has changed (integrates a lock for the variable) semaphore: a generalization of a lock, where there can be n threads operating in parallel (a lock is a semaphore with n=1) 2 Mutual Exclusion (mutex) A mutex is a simple lock that is shared between threads, used to protect critical regions of code or shared data structures. mutex m; mutex.lock() mutex.unlock() A mutex is often called a lock: the terms are mostly interchangeable When a thread attempts to lock a mutex: Currently unlocked: the thread takes the lock, and continues executing Currently locked: the thread blocks until the lock is released by the current lock- holder, at which point it attempts to take the lock again (and could compete with other waiting threads). Only the current lock-holder is allowed to unlock a mutex Deadlock can occur when threads form a circular wait on mutexes (e.g. dining philosophers) Places we've seen an operating system use mutexes for us: All file system operation (what if two programs try to write at the same time? create the same file?) Process table (what if two programs call fork() at the same time?) 3 lock_guard<mutex> The lock_guard<mutex> is very simple: it obtains the lock in its constructor, and releases the lock in its destructor.
    [Show full text]
  • Semaphores Semaphores (Basic) Semaphore General Vs. Binary
    Concurrent Programming (RIO) 20.1.2012 Lesson 6 Synchronization with HW support • Disable interrupts – Good for short time wait, not good for long time wait – Not good for multiprocessors Semaphores • Interrupts are disabled only in the processor used • Test-and-set instruction (etc) Ch 6 [BenA 06] – Good for short time wait, not good for long time wait – Nor so good in single processor system • May reserve CPU, which is needed by the process holding the lock – Waiting is usually “busy wait” in a loop Semaphores • Good for mutex, not so good for general synchronization – E.g., “wait until process P34 has reached point X” Producer-Consumer Problem – No support for long time wait (in suspended state) Semaphores in C--, Java, • Barrier wait in HW in some multicore architectures – Stop execution until all cores reached barrier_waitinstruction Linux, Minix – No busy wait, because execution pipeline just stops – Not to be confused with barrier_wait thread operation 20.1.2012 Copyright Teemu Kerola 2012 1 20.1.2012 Copyright Teemu Kerola 2012 2 Semaphores (Basic) Semaphore semaphore S public create initial value integer value semafori public P(S) S.value private S.V Edsger W. Dijkstra V(S) http://en.wikipedia.org/wiki/THE_operating_system public private S.list S.L • Dijkstra, 1965, THE operating system queue of waiting processes • Protected variable, abstract data type (object) • P(S) WAIT(S), Down(S) – Allows for concurrency solutions if used properly – If value > 0, deduct 1 and proceed • Atomic operations – o/w, wait suspended in list (queue?) until released – Create (SemaName, InitValue) – P, down, wait, take, pend, • V(S) SIGNAL(S), Up(S) passeren, proberen, try, prolaad, try to decrease – If someone in queue, release one (first?) of them – V, up, signal, release, post, – o/w, increase value by one vrijgeven, verlagen, verhoog, increase 20.1.2012 Copyright Teemu Kerola 2012 3 20.1.2012 Copyright Teemu Kerola 2012 4 General vs.
    [Show full text]
  • CSC 553 Operating Systems Multiple Processes
    CSC 553 Operating Systems Lecture 4 - Concurrency: Mutual Exclusion and Synchronization Multiple Processes • Operating System design is concerned with the management of processes and threads: • Multiprogramming • Multiprocessing • Distributed Processing Concurrency Arises in Three Different Contexts: • Multiple Applications – invented to allow processing time to be shared among active applications • Structured Applications – extension of modular design and structured programming • Operating System Structure – OS themselves implemented as a set of processes or threads Key Terms Related to Concurrency Principles of Concurrency • Interleaving and overlapping • can be viewed as examples of concurrent processing • both present the same problems • Uniprocessor – the relative speed of execution of processes cannot be predicted • depends on activities of other processes • the way the OS handles interrupts • scheduling policies of the OS Difficulties of Concurrency • Sharing of global resources • Difficult for the OS to manage the allocation of resources optimally • Difficult to locate programming errors as results are not deterministic and reproducible Race Condition • Occurs when multiple processes or threads read and write data items • The final result depends on the order of execution – the “loser” of the race is the process that updates last and will determine the final value of the variable Operating System Concerns • Design and management issues raised by the existence of concurrency: • The OS must: – be able to keep track of various processes
    [Show full text]
  • Lesson-9: P and V SEMAPHORES
    Inter-Process Communication and Synchronization of Processes, Threads and Tasks: Lesson-9: P and V SEMAPHORES Chapter-9 L9: "Embedded Systems - Architecture, Programming and Design", 2015 Raj Kamal, Publs.: McGraw-Hill Education 1 1. P and V SEMAPHORES Chapter-9 L9: "Embedded Systems - Architecture, Programming and Design", 2015 Raj Kamal, Publs.: McGraw-Hill Education 2 P and V semaphores • An efficient synchronisation mechanism • POSIX 1003.1.b, an IEEE standard. • POSIX─ for portable OS interfaces in Unix. • P and V semaphore ─ represents an integer in place of binary or unsigned integers Chapter-9 L9: "Embedded Systems - Architecture, Programming and Design", 2015 Raj Kamal, Publs.: McGraw-Hill Education 3 P and V semaphore Variables • The semaphore, apart from initialization, is accessed only through two standard atomic operations─ P and V • P (for wait operation)─ derived from a Dutch word ‘Proberen’, which means 'to test'. • V (for signal passing operation)─ derived from the word 'Verhogen' which means 'to increment'. Chapter-9 L9: "Embedded Systems - Architecture, Programming and Design", 2015 Raj Kamal, Publs.: McGraw-Hill Education 4 P and V Functions for Semaphore P semaphore function signals that the task requires a resource and if not available waits for it. V semaphore function signals which the task passes to the OS that the resource is now free for the other users. Chapter-9 L9: "Embedded Systems - Architecture, Programming and Design", 2015 Raj Kamal, Publs.: McGraw-Hill Education 5 P Function─ P (&Sem1) 1. /* Decrease the semaphore variable*/ sem_1 = sem_1 1; 2. /* If sem_1 is less than 0, send a message to OS by calling a function waitCallToOS.
    [Show full text]
  • Shared Memory Programming: Threads, Semaphores, and Monitors
    Shared Memory Programming: Threads, Semaphores, and Monitors CPS343 Parallel and High Performance Computing Spring 2020 CPS343 (Parallel and HPC) Shared Memory Programming: Threads, Semaphores, and MonitorsSpring 2020 1 / 47 Outline 1 Processes The Notion and Importance of Processes Process Creation and Termination Inter-process Communication and Coordination 2 Threads Need for Light Weight Processes Differences between Threads and Processes Implementing Threads 3 Mutual Exclusion and Semaphores Critical Sections Monitors CPS343 (Parallel and HPC) Shared Memory Programming: Threads, Semaphores, and MonitorsSpring 2020 2 / 47 Outline 1 Processes The Notion and Importance of Processes Process Creation and Termination Inter-process Communication and Coordination 2 Threads Need for Light Weight Processes Differences between Threads and Processes Implementing Threads 3 Mutual Exclusion and Semaphores Critical Sections Monitors CPS343 (Parallel and HPC) Shared Memory Programming: Threads, Semaphores, and MonitorsSpring 2020 3 / 47 What is a process? A process is a program in execution. At any given time, the status of a process includes: The code that it is executing (e.g. its text). Its data: The values of its static variables. The contents of its stack { which contains its local variables and procedure call/return history. The contents of the various CPU registers { particularly the program counter, which indicates what instruction the process is to execute next. Its state { is it currently able to continue execution, or must further execution wait until some event occurs (e.g. the completion of an IO operation it has requested)? CPS343 (Parallel and HPC) Shared Memory Programming: Threads, Semaphores, and MonitorsSpring 2020 4 / 47 What is a process? The chief task of an operating system is to manage a set of processes.
    [Show full text]
  • Software Transactional Memory with Interactions 3
    Software Transactional Memory with Interactions⋆ Extended Version Marino Miculan1 and Marco Peressotti2 1 University of Udine [email protected] 2 University of Southern Denmark [email protected] Abstract Software Transactional memory (STM) is an emerging ab- straction for concurrent programming alternative to lock-based synchron- izations. Most STM models admit only isolated transactions, which are not adequate in multithreaded programming where transactions need to interact via shared data before committing. To overcome this limitation, in this paper we present Open Transactional Memory (OTM), a pro- gramming abstraction supporting safe, data-driven interactions between composable memory transactions. This is achieved by relaxing isolation between transactions, still ensuring atomicity. This model allows for loosely-coupled interactions since transaction merging is driven only by accesses to shared data, with no need to specify participants beforehand. 1 Introduction Modern multicore architectures have emphasized the importance of abstractions supporting correct and scalable multi-threaded programming. In this model, threads can collaborate by interacting on shared data structures, such as tables, message queues, buffers, etc., whose access must be regulated to avoid inconsist- encies. Traditional lock-based mechanisms (like semaphores and monitors) are notoriously difficult and error-prone, as they easily lead to deadlocks, race con- ditions and priority inversions; moreover, they are not composable and hinder parallelism, thus reducing efficiency and scalability. Transactional memory (TM) has emerged as a promising abstraction to replace locks [5, 20]. The basic idea is to mark blocks of code as atomic; then, execution of each block will appear either as if it was executed sequentially and instantaneously at some unique point in time, or, if aborted, as if it did not execute at all.
    [Show full text]
  • Semaphores Cosc 450: Programming Paradigms 06
    CoSc 450: Programming Paradigms 06 Semaphores CoSc 450: Programming Paradigms 06 Semaphore Purpose: To prevent inefficient spin lock of the await statement. Idea: Instead of spinning, the process calls a method that puts it in a special queue of PCBs, the “blocked on semaphore” queue. 71447_CH08_Chapter08.qxd 1/28/09 12:27 AM Page 422 422 Chapter 8 Process Management The PCB contains additional information to help the operating system schedule the CPU. An example is a unique process identification number assigned by the system, labeled Process ID in Figure 8.18, that serves to reference the process. Sup- pose a user wants to terminate a process before it completes execution normally, and he knows the ID number is 782. He could issue a KILL(782) command that would cause the operating system to search through the queue of PCBs, find the PCB with ID 782, remove it from the queue, and deallocate it. Another example of information stored in the PCB is a record of the total amount of CPU time used so far by the suspended process. If the CPU becomes available and the operating system must decide which of several suspended processes gets the CPU, it can use the recorded time to make a fairFigure decision. 8.19 As a job progresses through the system toward completion, it passes through several states, as Figure 8.19 shows. The figure is in the form of a state transition diagram and is another example of a finite state machine. Each transition is labeled with the event that causes the change of state.
    [Show full text]
  • Low Contention Semaphores and Ready Lists
    View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Purdue E-Pubs Purdue University Purdue e-Pubs Department of Computer Science Technical Reports Department of Computer Science 1980 Low Contention Semaphores and Ready Lists Peter J. Denning T. Don Dennis Jeffrey Brumfield Report Number: 80-332 Denning, Peter J.; Dennis, T. Don; and Brumfield, Jeffrey, "Low Contention Semaphores and Ready Lists" (1980). Department of Computer Science Technical Reports. Paper 261. https://docs.lib.purdue.edu/cstech/261 This document has been made available through Purdue e-Pubs, a service of the Purdue University Libraries. Please contact [email protected] for additional information. LOW CONTENTION SEMAPHORES AND READY LISTS Peter J. Denning T. Don Dennis Jeffrey Brumfield Computer Sciences Department Purdue University W. Lafayette, IN 47907 CSD-TR-332 February 1980 Revised June 1980 Abstract. A method for reducing sema­ phore and ready list contention in mul­ tiprocessor operating systems is described. Its correctness is esta­ blished. Its performance is compared with conventional implementations. A method of implementing the ready list with a ring network is proposed and evaluated. This work was supported in part by NSF Grant MCS78­ 01729 at Purdue University. June 18, 1980 - 2 - THE PROBLEM Modern operating systems implement semaphores for synchron- izing multiple concurrent processes. Unless the primitive opera- tions for starting, stopping, and scheduling processes and for manipulating semaphores are well supported by the hardware, con- text switching and interprocess signaling can be major overheads [1]. To avoid these overheads, many operating systems use fast, but unreliable ad hoc methods for synchronizing processes.
    [Show full text]
  • Semaphores (Dijkstra 1965)
    Semaphores (Dijkstra 1965) n Semaphores have a non-negative integer value, and support two operations: n semaphore->P(): an atomic operation that waits for semaphore Semaphores & Monitors to become positive, then decrements it by 1 n semaphore->V(): an atomic operation that increments semaphore by 1, waking up a waiting P, if any. n Semaphores are like integers except: Arvind Krishnamurthy (1) non-negative values; Spring 2004 (2) only allow P&V --- can’t read/write value except to set it initially; (3) operations must be atomic: -- two P’s that occur together can’t decrement the value below zero. -- thread going to sleep in P won’t miss wakeup from V, even if they both happen at about the same time. Implementing semaphores Using interrupts P means “test” (proberen in Dutch) V means “increment” (verhogen in Dutch) class Semaphore { int value = 0; } n Binary semaphores: Semaphore::P() { n Like a lock; can only have value 0 or 1 (unlike the previous Disable interrupts; Semaphore::V() { “counting semaphore” which can be any non-negative integers) while (value == 0) { Disable interrupts; Enable interrupts; value++; Disable interrupts; Enable interrupts; } } n How to implement semaphores? Standard tricks: value = value - 1; Enable interrupts; n Can be built using interrupts disable/enable } n Or using test-and-set n Use a queue to prevent unnecessary busy-waiting n Question: Can we build semaphores using just locks? Using test&set How to use semaphores n Binary semaphores can be used for mutual exclusion: class Semaphore { int value = 0; initial value of 1; P() is called before the critical section; and V() is called after the int guard = 0; } critical section.
    [Show full text]
  • Synchronization
    Synchronization CS 4410 Operating Systems [R. Agarwal, L. Alvisi, A. Bracy, M. George, E. Sirer, R. Van Renesse] • Foundations • Semaphores • Monitors & Condition Variables 2 Semaphores • Definition • Binary Semaphores • Counting Semaphores • Classic Sync. Problems (w/Semaphores) - Producer-Consumer (w/ a bounded buffer) - Readers/Writers Problem • Classic Mistakes with Semaphores 3 What is a Semaphore? [Dijkstra 1962] Dijkstra introduced in the THE Operating System Stateful: • a value (incremented/decremented atomically) • a queue • a lock Interface: • Init(starting value) • P (procure): decrement, “consume” or “start using” • V (vacate): increment, “produce” or “stop using” No operation to read the value! Dutch 4410: P = Probeer (‘Try'), V = Verhoog ('Increment', 'Increase by one') 4 Semantics of P and V P(): P() { • wait until value >0 while(n <= 0) ; • when so, decrement n -= 1; value by 1 } V(): V() { • increment value by 1 n += 1; } These are the semantics, but how can we make this efficient? (doesn’t this look like a spinlock?!?) 5 Implementation of P and V P(): P() { • block (sit on Q) til n > 0 while(n <= 0) • when so, decrement value by 1 ; n -= 1; V(): } • increment value by 1 • resume a thread waiting on Q V() { (if any) n += 1; } Okay this looks efficient, but how is this safe? 6 Implementation of P and V P() { P(): acquire(&guard); • block (sit on Q) til n > 0 while(n <= 0) { waiting.enq(self); • when so, decrement value by 1 release(&guard); sleep(); acquire(&guard); V(): } • increment value by 1 n -= 1; release(&guard);
    [Show full text]
  • Parallel Programming Models III (Pdf)
    Shared Memory Programming Models III Stefan Lang Interdisciplinary Center for Scientific Computing (IWR) University of Heidelberg INF 368, Room 532 D-69120 Heidelberg phone: 06221/54-8264 email: [email protected] WS 15/16 Stefan Lang (IWR) Simulation on High-Performance Computers WS15/16 1/36 Shared Memory Programming Models III Communication by shared memory Semaphore rep. Reader-Writer problem PThreads Active Objects Stefan Lang (IWR) Simulation on High-Performance Computers WS15/16 2/36 Semaphore A semaphore is an abstraction of a synchronisation variable, that enables the elegant solution of multiple of synchronisation problems Up-to-now all programs have used active waiting. This is very inefficient under quasi-parallel processing of multiple processes on one processor (multitasking). The semaphore enables to switch processes into an idle state. We understand a semaphore as abstract data type: Data structure with operations, that fulfill particular properties: A semaphore S has a non-negative integer value value(S), that is assigned during creation of the Semaphore with the value init. For a semaphore S two operations P(S) and V(S) are defined with: P(S) decrements the value of S by one if value(S) > 0, otherwise the process blocks as long as another process executes a Voperation on S. V(S) frees another process from a Poperation if one is waiting (are several waiting one is selected), otherwise the value of S is incremented by one. Voperations never block! Stefan Lang (IWR) Simulation on High-Performance Computers WS15/16 3/36 Readers/Writers Problem ReadersLeser WritersSchreiber DatabaseDatenbank Two classes of processes, readers and writers, access a common database.
    [Show full text]